1. Field of the Invention
The present invention relates to a method of preparing a positive active material for a lithium secondary battery, and particularly, to a method of preparing a positive active material capable of enhancing cell capacity, cycle performance, safety property, and efficiency by preventing a side reaction between a positive active material and an electrolyte, and by enhancing electric conductivity of the positive active material.
2. Background of the Invention
As metal oxides and phosphor oxides positive active materials for lithium secondary batteries, LiCoO2, LiMn2O4, LiMnO2, LiNiO2, LiCO1/3Ni1/3Mn1/3O2, V6O13, V2O5, LiFePO4 or (PO4)3, are being commonly used. As a metal oxides and phosphor oxides negative active material for lithium secondary batteries, a transition metal oxide, Li4Ti5O12 is being commonly used. And, MnO2 is being commonly used as a positive active material for lithium primary batteries.
As the most representative positive active material for lithium secondary batteries, LiCoO2 is being widely used because of its layered structure. Owing to the layered structure of a hexagonal system which belongs to a space group (R-3m) where Li+ and Co3+ are alternately positioned at an octahedral site between an oxygen ion layer having a highest density and an adjacent layer thereto, intercalation and deintercalation for lithium ions are facilitated. Under these structural characteristics, the LiCoO2 has stable charge and discharge characteristics, high thermal stability, high specific capacity, and high electric conductivity.
However, the LiCoO2 has the following disadvantages.
Firstly, the LiCoO2 is more costly than other positive active materials such as LiNiO2 and LiMn2O4. Secondly, the LiCoO2 has a low concentration of lithium ions that can be intercalated thereto for maintenance of the structural stability during charge and discharge processes. During the charge process, lithium ions are separated from a lithium layer to form an empty space, and Co4+ ions having similar diameters to each other among Co4+ ions in Co-layers move to the empty space to fill the empty space. The Co4+ ions having moved to the empty space enhance stability of a hexagonal structure, and prevent a phase transition from a monoclinic structure to a hexagonal structure. However, the Co4+ ions hinder movement of the lithium ions, thereby accelerating polarization during the charge and discharge processes. This polarization becomes severe at a high voltage (>4.5V) where the lithium ions are separated from the lithium layer by 50% or more than.
Another problem occurring during the charge and discharge processes at a voltage more than 4.2V may include Co-dissolution due to a side reaction between an electrode and an electrolyte. As a voltage increases, Co and O are dissolved from each other due to instability of the layered structure. At the same time, an oxidation reaction and a dissolution reaction between a Li salt and an organic solution which consist of the electrolyte are accelerated. And, a component of the electrolyte, LiPF6 is dissolved to generate HF. And, the generated HF reacts with LiCoO2 to form a new film (Solid Electrolyte Interface: SEI) on the electrode. This may lower conductivity of the lithium ions very much to increase mass transfer resistance. And, a high polymer is generated on the surface of an anode through positive ion polymerization while Ethylene Carbonate (EC) is oxidized. Since the generated high polymer is an electric insulator, it increases charge transfer resistance.
In order to solve these problems of the conventional electrode active material, there have been proposed methods for doping other elements to the electrode active material such as LiCoO2 or LiNiO2. U.S. Pat. No. 5,292,601 discloses LixMO2(M is one or more elements among Co, Ni and Mn, and 0.5≦x≦1) as a positive active material for enhancing performance of LiCoO2. Japanese Patent Laid-Open No. 9-55210 discloses a surface-modified positive active material prepared by coating alkoxide of Co, Al or Mn on LiNiO2, and then by heat-treating the coated resultant. And, Japanese Patent Laid-Open No. 11-16566 discloses a lithium-based oxide as a surface-modified positive active material, the lithium-based oxide coated with metal and crystalline oxides of Ti, Bi, B, Cu, Si, Sn, Ga, W, Zr or Mo.
However, the above methods have the following problems. Firstly, an initial temperature at which the surface of a positive active material and an electrolyte solution react with each other, i.e., a temperature (Ts) at which oxygen coupled to metal of the positive active material is separated from the metal during a charge process is not sufficiently increased. Secondly, a heating value occurring during a dissolution process is not sufficiently decreased.
As a similar technique to the present invention, there has been proposed Korean Patent Laid-Open No. 2002-066548 relating to a positive active material prepared by forming an amorphous oxide coating layer on mixed lithium metal oxides. The Korean Patent discloses a method of preparing a positive active material having been surface-coated with an amorphous material, comprising synthesizing mixed lithium metal oxides powder, additionally synthesizing an amorphous material, mixing the mixed lithium metal oxides powder and the amorphous material with each other, and heat-treating the mixture at 600° C. for five hours. However, while the heat-treatment is performed at a high temperature, the coating layer, the mixed lithium metal oxides has a high probability to be changed to a crystalline layer. And, in the Korean Patent Laid-Open No. 2002-066548, it is assumed that the material of the coating layer is amorphous without verifying the formation of an amorphous phase. Furthermore, according to the Korean Patent Laid-Open No. 2002-066548, the mixed lithium metal oxides have a high probability to be presented in the form of a mixture that the mixed lithium metal oxides and the material of the coating layer are uniformly distributed, rather than the surface of the mixed lithium metal oxides is coated with the coating layer. Korean Patent Laid-Open No. 2007-027985 discloses an electrode active material having a double layer structure of a core-shell structure. However, both the core and the shell include lithium ions. A large number of patents and theses rather than the above patents mention a coating technique for a positive active material. However, they merely mention a coating technique by a crystalline material. That is, there have not been proposed any techniques capable of enhancing ion conductivity, and capable of preventing a side reaction between an electrolyte and a positive active material, and corrosion of metal oxides by implementing a sophisticated amorphous mixed metal-oxides coating using a thin film processing.